Since the launch of Opera 10.5 in March 2010, I've been using it as my primary browser, whether at work or at home.
Using Ubuntu at work, and a Windows netbook at home, I wanted a fast browser for my netbook and a coherent browsing experience on both operating systems. And this is where Opera 10.5 (and newer) fits perfectly.

Right now Opera is my main browser, but for me, as Opera without extension API has very little capabilities, the first of two browsers that does this is the winner and will be my main browser:

Opera: Extension API.
Firefox 4.x: Responsive UI.

Ready ? FIGHT !

Exactly. Opera's lack of an extension system makes the browser limited to only what Opera's developers choose to add.
This is something we can say also about IE 9. Very impressive work from Microsoft, but as Opera, no extension api.

Firefox is a great browser. Very complete, but its performance on limited computers (i.e, netbooks) makes us, users, in a difficult position to keep using it.
There is great expectations with version 4. Mozilla ought not to deceive us, and must deliver a light responsive product.